The invention relates to a method for cleaning the seat of a double seat valve, which has two serially arranged closing members that are moveable relative to each other, which prevent in the closed position of the double seat valve the overflow of fluids from one valve housing part of a valve housing into another, which delimit amongst each other a leakage cavity both in the closed as well as in the open position, which is connected with the area surrounding the double seat valve via a discharge bore, which is bordered by a tube shaft designed on the first closing member and leading out of the valve housing.
A seat-cleaning-capable double seat valve of the initially identified type for performing the method is known from WO 2007/054 134 A1 or the subsequent application US 2009/0044874 A1. In the case of this double seat valve, the dependently driven (relating to a perpendicular normal position) upper closing member, hereinafter referred to as the second closing member, on a second end section facing an independently driven, lower closing member, hereinafter referred to as the first closing member, has a recess with a cylindrical circumferential wall, which is flush with a cylindrical first seat surface, wherein the recess is dimensioned to receive, during the opening movement, a first end section and a radial first seal of the first closing member in a sealing manner, before the second closing member opens. The first closing member is thereby designed as a pusher piston with a radial first seal and the second closing member either as a pusher piston with a radially acting second seal or conically with an axially/radially acting second seal. The known double seat valve limits among other things the cleaning agent quantity during the respective seat cleaning. Its leakage outlet, which must also discharge these cleaning agent quantities into the area surrounding the double seat valve, is generally measured such that it complies with the requirements or respectively stipulations of the United States Food and Drug Administration of the USA (USFDA) in the “3-A Sanitary Standards for Double-Seat Mixproof Valves, Number 85-02 [1],” which require among other things that the most minimal passage cross-section of the leakage outlet is measured such that it at least corresponds with the passage cross-section of the largest tube conduit that can be connected to the double seat valve (requirement D14.2). Furthermore, in connection with the seat cleaning, additional requirements as per [1] are met, which indicate that the respectively closed seat region is not directly flowed against or pressurized in a pressure-increasing manner by the respectively generated seat cleaning flow (D14.5.2.1) and that the pressure in the closed seat region facing the leakage space must be equal to or less than the atmospheric pressure (D14.5.2.2).
The known double seat valve thus also fulfills other implicit requirements of the above standard as per [1], and namely those that no cleaning agent can pass through in the case of bigger seal defects or even the loss of one of the two seat seals in the course of the seat cleaning of the other closing member via the respective seal defect or respectively the seat region without seat seal. Under these conditions, the known double seat valve not only fulfills the requirement of limiting the cleaning agent quantity and avoiding a direct impact on the seat regions during the seat cleaning, but rather also the requirement of removing the seat cleaning flow to the greatest possible extent without swirling at first into the leakage cavity and from there into the surrounding area, without the respectively closed seat region being directly flowed against or pressurized in a pressure-increasing manner.
Direct impact is understood as all speed components from the respective seat cleaning flow perpendicularly directed at the walls bordering the seat region. It was namely shown that all direct impacts to this effect lead to a conversion of kinetic flow energy to static pressure. Depending on the angle of impact of the flow to the flowed-against wall or body surface, a branch flow with a so-called “branch flow line” results, wherein the latter divides the flow into two halves. The branch flow line itself flows towards the so-called “stagnation point” so that the speed is equal to zero at this point. The pressure increase resulting from this speed stoppage is also called “stagnation pressure”. The pressure-increasing mechanisms shown above, if effective, generate a leakage flow over the respective restrictor gap and the defective or no longer present seat seal.
While the aforementioned double seat valve according to WO 2007/054 134 A1 or the subsequent application US 2009/0044874 A1 solves the requirements of the standard as per [1] solely with flow-mechanical means and modes of action on the components of the double seat valve bordering the leakage cavity, DE 10 2007 038 124 A1 or the subsequent application US 2009/0065077 A1 suggests fulfilling the said requirements of the standard as per [1] through a separate third member, a so-called flow barrier element, arranged between the two closing members of the double seat valve and relatively moveable with respect to both. During ventilation of the one closing member and during supplying of the leakage cavity with cleaning agent, this flow barrier element shadows the at least one sealing element and/or the closing member seat of the other closing member, which is located in its closed position, vis-à-vis a direct impact by the cleaning agent entering the leakage cavity. As shown in the description of the aforementioned document, “shadowing” is to be understood such that the sealing element of the respective closing member located in the closed position is not supplied directly and thus with a higher flow speed with the cleaning agent, wherein it is permitted that the cleaning agent is mainly pressure-less and arrives in the area of the closing member seat or of the sealing element of the closing member located in the closed position at a low flow speed so that stagnation pressure cannot form there. As shown in the description, the flow barrier element does not have to rest in a completely sealing manner on the housing side; rather, it is preferably distanced from the housing by a small gap. Furthermore, it can be seen in FIGS. 1, 4 to 7 and 12 to 15 of WO 2007/054 134 A1 or US 2009/0044874 A1 that the leakage outlet fulfills the requirement of the start as per [1] according to D14.2, namely that the most minimal passage cross-section of the leakage outlet corresponds at least with the passage cross-section of the largest tube conduit that can be connected to the double seat valve.
WO 98/41786 A1 (page 11, line 24 to page 12, line 9) or the subsequent application U.S. Pat. No. 6,178,986 B1 (column 6, row 58 to column 7, row 11) already describes but does not claim an independent third member that is relatively moveable with respect to the two closing members of a seat-cleaning-capable double seat valve and is guided in a sealed manner in the cylindrical seating for the first closing member. This known embodiment differs from the object of the post-published DE 10 2007 038 124 A1 or US 2009/0065077 A1 thus mainly through the interaction between the third member, the flow barrier element and the associated cylindrical seating for the first closing member. While the older solution here provides a sealing by means of a radially acting seal in sliding engagement, for the newer solution, the third member does not have to rest in a completely sealing manner on the housing side; rather, it is preferably distanced from the housing by a small radial gap.
The flow barrier element according to DE 10 2007 038 124 A1 or the subsequent application US 2009/0065077 A1 fulfills the aforementioned requirement D14.5.2.1 and, in the case of corresponding dimensioning of the leakage outlet, also the requirement D14.2 as per [1] through its “shadowing” effect in its embodiment sealed on the housing side or even potentially unsealed. The requirement D14.5.2.2 apparently remains unfulfilled because the flow barrier element now appears within the discussed seat-cleaning-capable double seat valve in a significantly modified embodiment, as shown in company document Pentair Südmo Operating Instructions, BAA D 365it Complete PMO, Version 1.01, Double-seat valve type D 365it Complete PMO type D620 [2], published in November 2011 (201111) at URL http://www.suedomo.de/resources/images/790.
The enhanced function of the known flow barrier element designed as an annular body is described in DE 10 2010 046 137 A1, which was published after the company document [2]. The annular body subdivides in the ventilation position of at least one of the closing members together with it the leakage space into a first leakage space section and a second leakage space section. The annular body is designed such that in particular in the respective seat cleaning position generated cleaning medium can pass through the annular body from the first leakage space section into the second leakage space section. This transfer takes place such that the pressure in the second leakage space section is reduced with respect to the pressure in the first leakage space section and the cleaning medium arrives at the leakage outlet from the second leakage space section. Thus, in addition to the function of the shadowing of the seal or respectively of the closing member seat of the closing member located in its closing position, the annular body also takes on the function of the restriction of the respective seat cleaning flow. But this restriction is only possible and sufficient if the annular body is always sufficiently sealed on the housing side and rests respectively in a sealing manner on the ventilated closing member in the necessary manner.
Through this embodiment, it is possible, as illustrated by pages 14 and 25 of the company document [2] and FIG. 1 of DE 10 2010 046 137 A1 with the first closing member and the tube shaft connected with it in mind, to reduce significantly the leakage outlet formed in the tube shaft with respect to the object of DE 10 2007 038 124 A1 or US 2009/0065077 A1 and thus in a manner deviating from clause D14.2 of the standard as per [1]. This deviating design of the double seat valve is possible due to exemption clause D14.2.1.1 of the standard as per [1], which says that a leakage outlet that is reduced with respect to D14.2 is permitted if data is made available with the differently designed double seat valve, which shows that the maximum pressure between the valve seats of the double seat valve is less than or equal to the maximum pressure in a connection line provided with an unreduced leakage outlet between a comparable block valve and bleed valve, called “block and bleed” in the standards as per [1].
The double seat valve according to the company document [2] or DE 10 2010 046 137 A1 has the noteworthy advantage that the valve housing can be designed one to two nominal widths smaller than the design with a leakage outlet that is not reduced in cross-section and thus considerably more cost-effectively because, in the open position of the double seat valve, in which the tube shaft penetrates the connection opening between the valve housing parts, the passage cross-section of the annular gap between the tube shaft and the connection opening, which must correspond with the passage cross-section of the largest tube conduit that can be connected to the valve housing, can be realized without the above nominal width enlargement.
The double seat valve according to the company document [2] or DE 10 2010 046 137 A1 has the big disadvantage that, for one, the third member in the form of a flow barrier element sealed on the housing side in connection with the other characteristics of its arrangement in the leakage cavity and inclusion in the closing member configuration causes a complicated and thus problem-prone constructive structure of the double seat valve. Moreover, this additional built-in part in the leakage cavity with additionally necessary sealing means, corners and dead spaces is generally difficult to clean in the passage and thus questionable for sanitary reasons in the proper range of use. A sufficient restriction of the respective seat cleaning flow is also only ensured when this seat cleaning flow passes the systematically provided restriction points in the flow barrier element and is not guided more or less unrestricted in the bypass in the form-fitting connection between the latter and the closing member located in its seat cleaning position.
Experts are thus looking for a solution for how to realize the exemption clause D14.2.1.1 and the clause D14.5.2.2 of the standard as per [1] without additional built-in parts in the leakage cavity of a seat-cleaning-capable double seat valve, and namely with solely flow-mechanical means and modes of action on the previously present components delimiting the leakage cavity.
The object of the invention is to suggest a method for cleaning the seat of a double seat valve and to further develop a double seat valve of the generic type for performing the method, which, even if the cross-sectional surface of the discharge bore is smaller than that of the largest tube conduit that can be connected to the double seat valve, ensure that the seat cleaning flow is guided to the greatest possible extent without swirling into and out of the leakage cavity, and reliably avoid a pressure-increasing, direct impact on the seat regions.